1. Field of the Invention
This invention relates to a memory that utilizes magnetoresistance effect. More particularly, it relates to a magnetoresistance effect type memory requiring only a small power consumption at the time of reproduction, improved in memory characteristics and utilizable as an inexpensive memory adapted to computer peripheral equipment, the memory being preferable as peripheral circuits are made high-speed, and a reproduction method and a reproducing device which make use of such a memory.
2. Related Background Art
Memories used in computers or electronic instruments are under strong competition in their technical development. Techniques progress at a rapidly advancing rate, and various new memory devices are proposed. In recent years, giant magnetoresistance (GMR) effect has been discovered in magnetoresistive films holding a non-magnetic layer between ferromagnetic layers, and magnetic sensors and memories that utilize this phenomenon are attracting notice. In the following description, memories that utilize magnetoresistive films are generically called MRAM.
In the MRAM, a triple-layer structure having two ferromagnetic layers and a thin non-magnetic layer held between them forms a basic structural unit where information is recorded. Between the two ferromagnetic layers holding a non-magnetic layer between them, the state of xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is recorded by utilizing a phenomenon that resistance values differ between a case where their directions of magnetization are identical and a case where they are antiparallel.
When recorded information is read, an alternating magnetic field weaker than that at the time of writing is applied to cause only one ferromagnetic layer to change in its direction of magnetization, where changes in resistance values are measured to read the state of xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d. The MRAM has an advantage that it has a good resistance to radiations, is non-volatile in principle, is high-speed and is not limited in the number of times for writing, because the information is magnetically recorded. Appropriation of existing semiconductor techniques can afford to perform high-density recording with ease, and hence there is an expectation of its substitution for DRAM in future. For example, Japanese Patent Application Laid-open No. Hei6-243673 discloses a proposal relating to its utilization as a memory.
The operating principle of the MRAM is shown below. FIG. 5A illustrates the construction of an MRAM. The MRAM is so constructed as to have on a substrate a first magnetic layer 11, a non-magnetic layer 12, a second magnetic layer 13, an insulating layer 80 and a write wire (word wire) 51 in this order. The magnetoresistive film portion formed of combination of ferromagnetic layers with a non-magnetic layer may have a multi-layer structure.
Two ferromagnetic layers, the first magnetic layer 11 and second magnetic layer 13, are formed of combination of a soft magnetic material and a hard magnetic material. The soft magnetic material forms a reproducing layer from which information is read, and the hard magnetic material forms a memory layer in which information is stored. In the MRAM shown in FIG. 5A, the first magnetic layer 11 serves as a reproducing layer making use of the soft magnetic material, and the second magnetic layer 13 as a memory layer making use of the hard magnetic material. A buffer layer of SiN, Ta or the like may also be provided between the substrate and the first magnetic layer 11.
Recording operation of the MRAM is performed by changing the direction of magnetization of the memory layer second magnetic layer 13 by means of a magnetic field generated in the write wire.
FIG. 5B shows a case where xe2x80x9c0xe2x80x9d is written. A recording electric current is flowed through the write wire from the back to the front in the vertical direction as viewed on the drawing, whereupon a magnetic field is generated in the direction of arrows. When information is recorded, the magnetic field generated is made strong, so that the directions of magnetization not only of the reproducing layer first magnetic layer 11 but also of the memory layer second magnetic layer 13 are written in the rightward direction as viewed on the drawing. This state is xe2x80x9c0xe2x80x9d.
FIG. 5C shows a case where xe2x80x9c1xe2x80x9d is written. A recording electric current is flowed through the write wire from the front to the back in the vertical direction as viewed on the drawing, whereupon a magnetic field is generated in the direction of arrows. When recorded, the magnetic field generated is made strong, so that the directions of magnetization not only of the reproducing layer first magnetic layer 11 but also of the memory layer second magnetic layer 13 are written in the leftward direction as viewed on the drawing. This state is xe2x80x9c1xe2x80x9d.
On the other hand, when information is reproduced, reproducing electric current pulses weaker than those at the time of recording are flowed through the write wire sequentially in the both directions to reverse the magnetization of the reproducing layer, and a change in resistance at that moment is read, thus the reproduction is accomplished.
FIGS. 5D to 5G are a series of views showing the reproducing operation. In the state the xe2x80x9c0xe2x80x9d is kept recorded as shown in FIG. 5B, the directions of magnetization of the magnetic layers change as shown in FIG. 5D when first a reproducing electric current is flowed through the write wire from the front to the back in the vertical direction as viewed on the drawing, and change as shown in FIG. 5E when next the electric current is flowed in the opposite direction.
When first the reproducing electric current is flowed through the write wire as shown in FIG. 5D, from the front to the back in the vertical direction as viewed on the drawing, a weak magnetic field is generated in the direction of an arrow. At such a magnetic field strength, the reproducing layer first magnetic layer 11 reverses in magnetization, but the magnetization of the memory layer second magnetic layer 13 remains kept in the direction of xe2x80x9c0xe2x80x9d. When next the reproducing electric current is flowed through the write wire as shown in FIG. 5E, from the back to the front in the vertical direction as viewed on the drawing, a weak magnetic field is generated in the direction of an arrow. At such a magnetic field strength, the reproducing layer first magnetic layer 11 reverses in magnetization, but the magnetization of the memory layer second magnetic layer 13 remains kept in the direction of xe2x80x9c0xe2x80x9d.
Take note of the directions of magnetization of the two magnetic layers. When first the reproducing electric current is flowed through the write wire from the front to the back in the vertical direction as viewed on the drawing, the directions of magnetization of the first magnetic layer 11 and second magnetic layer 13 stand antiparallel. When next the reproducing electric current is flowed through the write wire from the back to the front in the vertical direction as viewed on the drawing, the directions of magnetization of the first magnetic layer 11 and second magnetic layer 13 stand parallel. Hence, in the course where electric current pulses are flowed in the two directions, the resistance of the write wire changes from a high resistance in an antiparallel state to a low resistance in a parallel state. The state where resistance values change from a high resistance to a low resistance in this way is read to be xe2x80x9c0xe2x80x9d.
On the other hand, in the state the xe2x80x9c1xe2x80x9d is kept recorded as shown in FIG. 5C, the directions of magnetization of the magnetic layers change as shown in FIG. 5F when first a reproducing electric current is flowed through the write wire from the front to the back in the vertical direction as viewed on the drawing, and change as shown in FIG. 5G when next the electric current is flowed in the opposite direction.
When first the reproducing electric current is flowed through the write wire as shown in FIG. 5F, from the front to the back in the vertical direction as viewed on the drawing, a weak magnetic field is generated in the direction of an arrow. At such a magnetic field strength, the reproducing layer first magnetic layer 11 does not reverse in magnetization, and also the magnetization of the memory layer second magnetic layer 13 remains kept in the direction of xe2x80x9c1xe2x80x9d. When next the reproducing electric current is flowed through the write wire as shown in FIG. 5G, from the back to the front in the vertical direction as viewed on the drawing, a weak magnetic field is generated in the direction of an arrow. At such a magnetic field strength, the reproducing layer first magnetic layer 11 reverses in magnetization, but its strength is insufficient for changing the magnetization of the memory layer second magnetic layer 13, which remains kept in the direction of xe2x80x9c1xe2x80x9d.
Take note of the directions of magnetization of the two magnetic layers. When first the reproducing electric current is flowed through the write wire from the front to the back in the vertical direction as viewed on the drawing, the directions of magnetization of the first magnetic layer 11 and second magnetic layer 13 stand parallel. When next the reproducing electric current is flowed through the write wire from the back to the front in the vertical direction as viewed on the drawing, the directions of magnetization of the first magnetic layer 11 and second magnetic layer 13 stand antiparallel. Hence, in the course where electric current pulses are flowed in the two directions, the resistance of the write wire changes from a low resistance in a parallel state to a high resistance in an antiparallel state. The state where resistance values change from a low resistance to a high resistance in this way is read to be xe2x80x9c1xe2x80x9d.
Thus, whether the information recorded is xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d can be recognized by reading the changes in resistance that are caused when weak electric current pulses are flowed through the write wire. This recording reproduction method enables high-speed drive in non-volatile and non-destructive manner, and hence ideal memory characteristics can be expected. Methods of detecting changes in magnetoresistance electrically at the time of the above reproduction are proposed in variety, and are roughly grouped into absolute detection, in which their extent is compared by the value of resistance itself, and differential detection, in which judgement is made on whether the changes in resistance that are caused when electric currents are shifted in two directions are in the direction of increase or in the direction of decrease.
In the foregoing description of operation, a method in which memory is recorded and reproduced using the write wire. The write wire, however, is not essential as a constituent of the MRAM. In some structure, any adjacent other wiring may be appropriated to generate the magnetic field with which the magnetization of ferromagnetic layers is reversed.
From the viewpoint of materials used and mechanism of magnetoresistance, construction of the MRAM can be classified into a spin dependent scattering type making use of a metal non-magnetic layer as an intermediate layer, a spin valve type where the direction of magnetization of one ferromagnetic layer is fixed with an antiferromagnetic layer, a spin dependent tunneling type making use of an insulator non-magnetic layer, and besides a granular type where fine particles of a magnetic material are dispersed in the non-magnetic layer, and a CMR (colossal magnetoresistance) type making use of a perovskite oxide film.
In the spin dependent scattering type, the non-magnetic layer is formed as a Cu or the like metal layer and GMR takes place on account of spin dependent scattering between two magnetic layers. More specifically, when the direction of magnetization of the magnetic layers are parallel, electrons having a spin in the direction opposite to magnetization are scattered but electrons having a spin in the same direction as magnetization are not scattered, resulting in a low resistance on the whole. When conversely the direction of magnetization of the magnetic layers are antiparallel, both the electrons having a spin in the same direction as magnetization and the electrons having a spin in the direction opposite to magnetization are scattered, resulting in a high resistance on the whole. In that case, an MR (magnetoresistance) ratio of about 5% to about 10% is obtainable at room temperature, which is greater than the magnetoresistance effect which depends on the direction of electric current and magnetization, but is smaller than that of a spin dependent tunneling type.
The spin valve type is the same as the spin dependent scattering type in principle, but differs therefrom in that an antiferromagnetic layer is combined as one ferromagnetic layer to pin up the direction of magnetization. The direction of magnetization of the other magnetic layer can freely be rotated. Plotting a magnetization curve, it has a shape asymmetric depending on the direction of magnetization, which linearly changes from a low resistance to a high resistance in the vicinity of the zero magnetic field, providing a structure suited for magnetic sensors which detect a very weak magnetism. At present, this type has been put into practical use as read sensors of hard disks.
In the spin dependent tunneling type, the non-magnetic layer is formed of an insulator, where electrons pass through the insulator by tunneling to move across the two magnetic layers, and the magnetoresistance effect takes place in the form that depends on a difference in state and density of spinning electrons. More specifically, when the directions of magnetization of the magnetic layers are parallel, electrons having an up spin can come by tunneling into the state of up spin where the other ferromagnetic layer is vacant, and electrons having a down spin into the state of down spin where the other ferromagnetic layer is vacant. Hence, the difference in state and density of spinning electrons becomes small to provide a low resistance. When conversely the directions of magnetization of the magnetic layers are antiparallel, both the electrons having an up spin and the electrons having a down spin can not move by tunneling, and hence the difference in state and density of spinning electrons becomes large to provide a high resistance. In that case, an MR ratio of about 10% to about 30% is obtainable at room temperature, which is greater than that of the spin dependent scattering type. However, because of the structure interposing the insulator, the device resistance itself is greater than that of the spin dependent scattering type. Utilizing this phenomenon of spin dependent tunneling, magnetoresistance films formed in the spin valve type using the antiferromagnetic film are energetically studied as those for future hard-disk read sensors.
In the granular type, there are two types, a spin dependent scattering type making use of a metal as the non-magnetic layer and a spin dependent tunneling type making use of an insulator. In the spin dependent scattering type and spin dependent tunneling type described previously, a role is clearly allotted for each layer, whereas, in the granule type, what greatly differs is that the GMR takes place in the form that depends on spins of individual fine magnetic particles dispersed in a matrix. In a spin dependent tunneling type of a Co/AlOx system, an MR ratio of about 8% is obtainable at room temperature.
In the CMR type, there are a type in which a Mn oxide having a perovskite structure is held between perovskite Mn oxides having a higher spin polarization rate, and a type in which a laminar structure in perovskite is utilized as a tunnel junction. The CMR type has so high a spin polarization rate that an MR ratio of as high as 400% is obtainable at a very low temperature.
Magnetic materials used in the MRAM can be classified by direction of magnetization, into an in-plane magnetization film type having a magnetizing component that is parallel to the film plane and a perpendicular magnetization film type having a magnetizing component that is perpendicular to the film plane. Ferromagnetic materials such as NiFe and Co are those of the in-plane magnetization film type the direction of magnetization of which is parallel to the film plane. Such in-plane magnetization films, however, have a problem of the phenomenon of curling of magnetization because magnetic poles may approach to each other as magnetic materials becomes finer, resulting in a great antimagnetization. Once the curing occurs, it becomes difficult to distinguish the direction of magnetization. Accordingly, in the MRAM making use of an in-plane magnetization film, in order to provide shape anisotropy, the ferromagnetic layers serving as a memory cell must be made to have the shape having a major axis (e.g., a rectangular shape) when viewed on plane. It is estimated that the ratio of major axis/minor axis of such a rectangle must be at least twice. Hence, in order to prevent the phenomenon of curing, the memory cell is limited in size to provide an inhibitory factor for making the degree of integration higher.
Meanwhile, when ferrimagnetic materials comprised of rare earth-transition metals such as TbFe, TbFeCo and GdFe are used as materials of ferromagnetic layers, these magnetic materials have a high perpendicular magnetization anisotropy. Hence, depending on layer thickness and composition, they provide perpendicular magnetization films having magnetization in the direction perpendicular to the film plane. In the case of perpendicular magnetization films, the direction of magnetization is the film-plane vertical direction that forms the greatest antimagnetic field in shape, thus they have already overcome the maximum coefficient of antimagnetic field at the time they exhibit the perpendicular magnetization anisotropy. Namely, it is unnecessary to make memory cells rectangular as in the case of in-plane magnetization films, and the memory cells can be made to have the shape of a square plane. In addition, making a device minute makes its planar area smaller, compared with layer thickness direction which is the readily magnetizable axis. Hence, from the viewpoint of shape anisotropy, this tends to make the curling of magnetization less occur. Thus, the perpendicular magnetization films are more advantageous than the in-plane magnetization films in order to make higher the degree of integration of memory cell areas.
The direction of electric current is roughly grouped into CIP (current in plane), which is parallel to film plane, and CPP (current perpendicular to plane), which is perpendicular to film plane, in accordance with the manner of flowing of electric current to the MRAM or the manner of disposing electrodes. FIGS. 6A and 6B show respectively corresponding electrode structures.
As shown in FIG. 6A, the CIP is a structure wherein sense layers are provided on both sides of a memory cell consisting of first magnetic layer 81/non-magnetic layer 82/second magnetic layer 83. Sense electric current flows in parallel to the film plane. In FIG. 6A, one of the sense layers 84 is illustrated by dotted lines. In the CIP, the spin dependent scattering type magnetoresistance film is used. In such a case, one cell has a resistance of about 10 xcexa9 as sheet resistance, and a sense wire has a sheet resistance of 0.05 xcexa9. Also, the rate of magnetoresistance change is about 5% to about 10%, which is smaller than that of the spin dependent tunneling type. When in CIP structure a large number of cells are connected in series to sense wires and signals are detected at the both ends thereof, changes in resistance for one cell are used as signals, with respect to the resistance synthesized by adding up resistance values of many cells connected, and hence it is not easy to achieve a high SN.
As shown in FIG. 6B, the CPP is a structure wherein sense layers are provided on the top and bottom of a memory cell consisting of first magnetic layer 91/non-magnetic layer 92/second magnetic layer 93. Sense electric current flows in the direction perpendicular to the film plane. In FIG. 6B, the upper sense layer 94 is illustrated by dotted lines. The lower sense layer 95 is illustrated by continuous lines. In the CPP, the spin dependent tunneling type magnetoresistance film may preferably be used. In such a case, one cell has a resistance ranging form several kxcexa9 to tens of kxcexa9, having a larger value of resistance than sense wires. Also, the rate of magnetoresistance change is about 10% to about 30%, which is larger than that of the spin dependent scattering type. More specifically, sufficiently great changes in resistance can be obtained even when the magnetoresistance films are connected to sense wires, so that a high SN can be achieved.
In this CPP structure, the cell is disposed at an intersection of sense wires. Hence, when many cells are disposed, the cells are each connected in parallel. In such construction, when resistance of a certain cell is to be detected, electric current is flowed through a sense wire intersecting that cell, whereby the resistance can be detected without being affected by other cells so much. Hence, a higher SN can be achieved than in the CIP structure. Thus, the cells that can be connected to one row of sense wires can be in a larger number and a large-scale matrix can be formed with greater ease in the CPP structure than in the CPP structure. Namely, considering that a large number of memory cells are arranged as memory devices to drive them, the CPP structure is more advantageous than the CIP structure.
Where differential detection is utilized in the MRAM, changes in resistance at the time positive and negative electric currents are alternately flowed are differentially detected to recognize xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d. A bipolar power source is necessary for generating the positive and negative electric currents. In order to materialize a high-speed bipolar function, it is important to operate several switches at a high speed and accurate timing in order to reverse the direction of electric current. Any deviation of timing causes ringing or overshoot in electric current waveforms. These may cause misoperation, and must be prevented as far as possible. To prevent ringing or overshoot, it is indispensable to optimize power source circuits taking account of wiring capacity, load resistance and so force which can be delay factors. This require a large space to be occupied by such power source circuits concurrently with connection of more transistors for materializing the function of switching, bringing about a problem in making higher the degree of integration as memories. This causes a difficulty in making the degree of integration higher, and hence may also be a factor which makes a memory require a high unit cost per bit.
Recently, as a field where solid-state memories are utilized, MP3 players attract notice in place of Walkman type headphone stereophonic players utilizing tape mediums. When applied to MP3 players, the solid-state memories can fully show their advantages from the viewpoint of resistance to shocks, durability, making compact and so forth. In addition, they require no mechanical drives and can make the most of an advantage of low power consumption. Also, it is supposed that reproduction-only sources making use of solid-state memories can be supplied in place of reproduction-only sources supplied for CDs and MDs.
The MRAM, too, is considered to have a reasonable number of chances where it is utilized for the needs on reproduction-only. To achieve its wide spread, the space and cost for the bipolar power sources exclusively used for reproduction as stated above can not be disregarded.
The above problems are solved if the signal reproduction of the MRAM can be materialized by flowing any one of positive and negative electric current pulses. If any bipolar function becomes unnecessary in a power source circuit to be added to conductor wires, when information is reproduced, the circuit construction can be simplified and also a cost reduction can be achieved. Also, any limitations on making the degree of integration higher can be eliminated, so that it becomes possible to further lower the unit cost per bit with ease. Although there is such a demand, any signal reproduction using any one of positive and negative electric currents has not been materialized.
The present invention solves the problems discussed above. Accordingly, an object of the present invention is to provide an MRAM which enables reproduction without applying positive and negative electric current pulses, and provide a method of reproducing information from such an MRAM by applying only a positive or negative electric current pulse, and a reproducing device used in such a method.
Another object of the present invention is to materialize an inexpensive memory having a high MRAM characteristics and adapted to computer peripheral equipment, the memory being preferable as peripheral circuits are made high-speed.
The above objects can be achieved by a magnetoresistance effect type memory in and from which information is recorded and reproduced by utilizing a magnetoresistance effect, the memory comprising;
a substrate;
a magnetoresistance film provided on the substrate, which comprises a reproducing layer, a memory layer and a non-magnetic layer provided between the reproducing layer and the memory layer; and
a magnetization-fixing layer provided on the substrate, which orients a magnetization of the reproducing layer to one direction.
The above objects can also be achieved by a recording-reproducing method for the above magnetoresistance effect type memory having a conductor wire in its vicinity, the method comprising the steps of;
applying any one of positive and negative electric current pulses to the conductor wire; and
detecting a change in resistance of the magnetoresistance film to reproduce information recorded in the memory layer.
The above objects can still also be achieved by a recording-reproducing device for recoding and reproducing information in and from the above magnetoresistance effect type memory having a conductor wire in its vicinity, the device comprising;
a means for supplying a uni-directional electric current to the conductor wire; and
a means for detecting a change in resistance of the magnetoresistance film to reproduce information recorded in the memory.
These will be described in detail in Description of The Preferred Embodiments given later.